1. Field of the Invention
The present invention relates to spectroscopic analysis of individual regions of inhomogeneous samples. The regions to be analyzed are identified, selected and imaged at a high spatial resolution using scanning probe microscopy.
2. Description of the Related Art
Techniques for the photothermal characterization of solids and thin films are widely used, as is described by D. P. Almond and P. M. Patel, "Photothermal Science and Techniques", Chapman and Hall (London and New York, 1996). Recently the ability to add spatial resolution to these techniques has become of technical interest in many fields: one example is the general area of electronic and optical devices. However, most methods commercially available suffer from the limitations imposed by the finite optical wavelengths of the detection systems used. For example, in practice the spatial resolution of the popular but expensive technique of Fourier transform infrared microscopy is seldom better than five to ten micrometers.
Most conventional methods of thermal imaging employ an energy beam that emerges from a small source and spreads out according to the rules of diffraction. The extent of this spreading is normally governed by the wavelength associated with the energy flux. However, if the sample is within the "near-field" region, i.e., significantly less than one wavelength away from the source, then a greatly reduced beam diameter can be achieved. In fact, when the sample is less than one wavelength away from the source, the diameter of the beam is not much larger than the size of the source itself. This principle is applied in Scanning Probe Microscopy. In Scanning Probe Microscopy, a sharp probe is brought into close proximity to the surface of a sample. Some probe/sample interaction takes place. This interaction is monitored as the probe is scanned over the surface. An image contrast is then computer-generated. The image contrast represents variations of some property (e.g., physical, mechanical, chemical) of the sample across the scanned area. One such probe microscope is the Atomic Force Microscope (AFM). In conventional AFM, the height of a probe above the surface being scanned is controlled by a feedback system. The feedback system keeps the force between the probe and the surface of the sample constant. The probe height is monitored, and provides the data that is used to create image contrast which represents the topography of the scanned area.
The use of miniature thermocouple probes and other near-field devices, as part of a scanning probe microscopy system, allows the limitations imposed by diffraction to be overcome, so that near-field scanning photothermal spectroscopy has become recognized as a research technique, as described, for example, by C. C. Williams and H. K. Wickramasinghe in "Photoacoustic and Photothermal Phenomena", P. Hess and J. Petal (eds.), Springer (Heidelberg, 1988). In their device, the probe is a specially made coaxial tip that forms a fine thermocouple junction. This probe provided a spatial resolution of the order of tens of nanometers. The sample is either heated using a laser or the probe, or the sample is heated electrically. The feedback system maintains the probe temperature constant (instead of maintaining the force constant), by varying the probe height as necessary.
J. M. R. Weaver, L. M. Walpita and H. K. Wickramasinghe in Nature, Vol. 342 pp. 783-5 (1989), described experiments that were similar to the Williams and Wickramasinghe experiments, except that the thermocouple junction was formed by contact between a scanning tunnelling microscopy probe (formed from one single electrical conductor) and an electrically conducting sample. They used this setup to perform optical absorption microscopy and spectroscopy with nanometer-scale spatial resolution. Types of image obtained included an electron tunnelling image that was sensitive to variations in surface topography, and a thermal image that was sensitive to variations in optical absorption properties and thermal properties of the sample-substrate system.
In another article, published in Soc. Photo. Instrum. Engrs. Vol. 897, pp. 129-134 (1988), C. C. Williams and H. K. Wickramasinghe used a near-field thermal probe in passive mode to measure photothermally-induced temperature variations in an electron beam-generated grating structure. They suggested that near field thermal and photothermal microscopy would find application in optical absorption spectroscopy at sub-optical lateral resolutions, and for measurement of exothermic and endothermic processes on a small scale.
Further developments in this field are described by E. Oesterschulze, M. Stopka and R. Kassing, Microelectronic Engineering vol. 24 pp. 107-112 (1994), and the field has been reviewed by A. Majumdar, K. Luo, Z. Shi and J. Varesi in Experimental Heat Transfer vol. 9 pp. 83-103 (1996). In "Thermal Imaging Using the Atomic Force Microscope," Appl. Phys. Lett., vol. 62, pp. 2501-3 (1993), Majumdar, et al. describe a technique for thermal imaging that uses a simpler design of thermocouple tip, than that disclosed by Williams and Wickramasinghe. They also implemented standard atomic force microscopy feedback to maintain tip/sample contact. R. B. Dinwiddie, R. J. Pylkki and P. E. West "Thermal Conductivity Contrast Imaging with a Scanning Thermal Microscope," Thermal Conductivity 22, T. W. Tsong (ed.) (1994), describe the use of a probe in the form of a tiny platinum resistance thermometer. U.S. Pat. No. 5,441,343 to Pylkki et al. (the "'343 patent") discloses the thermal sensing probe for use with a scanning probe microscope, in which the contact force of the probe is maintained at a constant level as the probe is scanned across the surface of the sample.
Also relevant is the recently developed technique for localised chemical fingerprinting by means of thermal analysis performed in a scanning thermal microscope. This has been described in U.S. Pat. No. 5,248,199 to Reading et. al (the "'199 patent") and U.S. patent application Ser. No. 08/837,547 to Hammiche et. al (the "'547 application"), both of which are incorporated herein by reference. It has also been described in the following publications: A. Hammiche, H. M. Pollock, M. Song and D. J. Hourston, Measurement Science and Technology 7, 142-150 (1996); A. Hammiche, H. M. Pollock, D. J. Hourston, M. Reading and M. Song, J. Vac. Sci. Technol. B14 (1996) 1486-1491; A. Hammiche, M. Reading, H. M. Pollock, M. Song and D. J. Hourston, Rev. Sci. Instrum. 67,4268 (1996); and H. M. Pollock, A. Hammiche, M. Song, D. J. Hourston and M. Reading, Journal of Adhesion, Vol. 67, pp. 193-205 (1998). That invention relates to the measurement of the thermal properties of materials using a miniaturized resistive thermal probe, and more particularly, to performing localized thermal analysis experiments whereby calorimetric information is obtained from volume of materials of the order of a few cubic microns, whereas in conventional bulk calorimetry data is obtained for volumes of material of a few cubic millimeters. In the course of this work, a means to perform subsurface depth profiling and imaging using thermal waves was also developed.
The other aspect of that invention relates to modulating the temperature of the probe to generate evanescent thermal waves in a material under study to thereby generate sub-surface images. It allowed for application of a modulated temperature differential scanning calorimetry technique, such as described in U.S. Pat. No. 5,224,775 to Reading, et al. (the "'775 patent"), which has been conventionally used to perform bulk thermal analysis experiments of a sample material, to microscopy using two highly miniaturized resistive probes, developed by the Topometrix Corporation and described in the '343 patent, in a differential arrangement. A sample probe, attached to a Scanning Probe Microscope, is positioned at a desired location on the surface within the field of view. Localized calorimetry is then performed at that position by inducing and detecting localized phase transitions. This is achieved by ramping the temperature of the probe by passing an appropriate current through it. To that temperature ramp a small temperature oscillation is superimposed by adding a modulated current into the probe. By scanning over the surface of the sample, contrast can be developed corresponding to particular locations on the sample to create an image of the thermal properties of the sample at the particular locations.
The probe, developed by the Topometrix Corporation, is an elongated loop of Wollaston wire, shaped in the form of a cantilever whose end forms the resistive element. The resistance of that element varies with temperature. Conversely, its temperature can be set by passing a current of appropriate value through it. A mirror is attached across the loop allowing for the contact force of the element on the sample to be held constant, as in conventional atomic force microscopy while the probe is scanned across the surface of the sample.
The probe is used as a highly localised heat source by passing a current through it. Its temperature is set constant and/or time varying. As the probe is brought close to the surface of a sample, heat will flow from the probe to the sample. The amount of heat flowing will vary according to various properties of the sample at the location under the probe. This varying heat flow causes the temperature of the resistive element to change, thereby changing its resistance. A feedback circuit is preferably used to sense the change in the probe resistance (and therefore its temperature) and increase the amount of current flowing through the probe to bring it back to its original resistance value (and therefore its set temperature).
A differential signal is then monitored, either directly or through a lock-in amplifier. The differential signal is used to either (1) to produce localized analysis plots of amplitude and phase data versus temperature that provide calorimetric information at a specific position on the sample, or (2) to construct an image whose contrasts represent variations in thermal conductivity and/or diffusivity across a scanned area. In the second embodiment, the time-varying current through the resistive elements generates thermal waves in the sample. The modulation frequency of the time-varying current is functionally related to the depth below the surface of the sample at which an image of the sample is desired. A sub-surface image is thus generated. The depth of material below the sample surface that is contributing to the image can be controlled by suitably choosing the temperature modulation frequency. As described in Almond, et al., "Photothermal Science and Techniques," page 15, Chapman and Hall (London 1996), which is hereby incorporated by reference in its entirety, the penetration depth is proportional to the square root of the thermal diffusivity of the sample divided by the frequency of the applied temperature wave.
It would be advantageous to be able to extend such chemical fingerprinting techniques to give true chemical analysis. Previous work on optical absorption spectroscopy combined with near-field microscopy has either been limited to the study of electrically conductive samples or has been restricted to the use of individual wavelengths of the incident light. Moreover, no reliable way has been described of deconvoluting spatial variations in thermal properties from the local variations in infrared absorption which are the key to localised spectroscopic analysis. Thus, so far there has been no report of such techniques having been applied to chemical analysis by means of spectroscopy at high spatial resolution, and this is the subject to which the present invention relates. Each publication, patent and patent application referred to herein is hereby incorporated by reference in its entirety herein.